A major challenge in cooling electric vehicles as well as mechanical and electrical systems, subsystems and components for electric vehicles, is formulating fluids with satisfactory heat transfer performance in specific devices. In particular, the challenge in heat transfer fluids is formulating fluids with satisfactory heat transfer performance in specific devices, and also having compatibility with electric vehicle components and materials.
The removal of heat from electric vehicle components such as batteries and electric motors during electric vehicle operation is commonly done using aqueous heat transfer fluids, which indirectly remove heat from the hot surfaces. As electric vehicle technology evolves to comprehend longer battery ranges, shorter recharging times, and higher vehicle power, there will be benefits associated with direct cooling of hot components, which is not possible with aqueous heat transfer fluids.
For example, direct cooling is significantly more efficient in emergency situations like run away reactions inside battery cells. The faster heat removal allows for improved thermal management where battery cells will not reach critical temperatures that can lead to irreversible battery fires. Indirectly cooled systems (e.g., water/glycol) are limited by the thermal conductivity of the jacket. Fast heat removal is a major benefit of a directly cooled system. Fast heat removal is also needed, for example, during super fast charging of lithium ion batteries.
In many electric vehicle applications, the performance of a heat transfer fluid is governed both by its ability to remove heat from hot surfaces and by the amount of power required to circulate the heat transfer fluid. An ideally-suited heat transfer fluid will maximize heat removal and require minimum power to circulate the fluid.
A Mouromtseff equation is used in the art for comparing the impact of heat transfer fluid properties on the resulting heat transfer coefficient. The Mouromtseff equation for turbulent flow systems is defined as follows: k0.67*ρ0.8*cp0.33*μ−0.47; and for laminar flow systems is defined as follows: While the use of the Mouromtseff equation provides a convenient method for quickly comparing heat transfer fluids, its use has a number of short comings. For example, use of the Mouromtseff equation implies that heat transfer in the physical situation in which the fluid is to be used is limited by heat transfer. In some situations (for example, if large heat transfer areas exist at the element to be cooled and the heat rejection site), heat conveyance by the circulating fluid may dominate. In such a situation, the actual mechanism of local heat transfer, and therefore fluid property impacts on that heat transfer, become significantly diminished. In these applications, while the Mouromtseff equation may be indicating something about the fluid, what it is indicating is significantly less relevant to its heat transfer performance.
Despite advances in heat transfer fluid formulation technology in electric vehicles, there exists a need for formulating fluids with satisfactory heat transfer performance in specific devices. Also, there is a need for heat transfer fluid formulations having compatibility with specific device components and materials. Further, there exists a need for heat transfer fluids that can maximize heat removal and require minimum power to circulate. Still further, an improved method for quickly and conveniently comparing heat transfer fluids and heat transfer that addresses operating variables, in addition to heat transfer fluid properties, in heat conveyance dominated situations, is needed.